ECHINODERMATA 659 



processes of the oral side of the body. The plates of the theca are 

 hexagonal, and bear pores, either in pairs (diplopores) or in diamond 

 patterns (pore-rhombs). The members of the group fall into two 

 divisions as follows : 



DiPLOPORiDA, with diplopores and epithecal grooves. Eucystis, 



Ordovician. 

 Rhombifera, with pore-rhombs and exothecal grooves. Echino- 



sphaera, Ordovician; Lepadocrinus, Silurian. 



Blastoidea. Highly organized forms, typically with ovoid, stalked 

 body. Certain plates of the theca (basals, radials, deltoids, lancets, etc.) 

 are uniform in arrangement throughout the group. The ambulacra 

 are bordered by rows of pinnule-like brachioles, and at the sides of the 

 ambulacral grooves run elongated internal pouches, the hydrospires, 

 which open to the exterior at the oral end by spiracles. Pentremites 

 Carboniferous. 



